1. Technical Field
The present disclosure relates to a semiconductor quantum cascade lasers. More specifically, the present disclosure relates to a nitride semiconductor quantum cascade laser that includes a gallium nitride based material.
2. Description of the Related Art
Recently, for solid-state lasers operating in wavelength ranges in which electromagnetic wave radiation by inter-band transition is difficult, quantum cascade lasers (QCLs) that utilize inter-subsub band transition of conduction carriers without jumping across a band gap are regarded as promising. QCLs are expected to be put into practical use due to their properties, such as ultra-compactness, high efficiency/high output power, narrow linewidth, long life, continuous wave operation, inexpensiveness, and high durability; and the development of QCLs has been in progress for mid-infrared and terahertz frequency ranges. In particular, when terahertz quantum cascade lasers (THz-QCLs) capable of lasing in a terahertz frequency range of 0.1 THz to 30 THz are realized, the THz-QCLs are expected to be applied to such fields as medical imaging, security check, and high-speed wireless communication. However, lasing operation of THz-QCLs has only been reported in a frequency range of 1.2 to 5.2 THz or a range over 12 THz. That is, lasing operations in a frequency range close to 1 THz, or in a frequency range between 5.2 THz and 12 THz has never been reported. It is an important issue to realize a THz-QCL capable of lasing in such frequency ranges, or in the unexplored frequency ranges.
For conventional materials of THz-QCLs, GaAs-, InP-, and InSb-based semiconductors have been employed. However, even when these materials are employed, it is difficult to realize a THz-QCL of frequency range of 5 to 12 THz. This is because, energy bands of scattering through Froehlich interaction between electrons and longitudinal-optical (LO) phonons for these materials have an overlap with a frequency range of 5 to 12 THz. For example, the LO-phonon energy ELO for GaAs is 36 meV, which is equivalent to 9 THz. In addition, population inversion is degraded due to the fact that refilling lower lasing level with electrons, called thermal backfilling, is likely to occur with GaAs-, InP-, and InSb-based semiconductors, which is also disadvantageous to lasing operation.
It is expected that employing a nitride semiconductor, in place of the above-mentioned conventional materials such as GaAs, allows a THz-QCL of a frequency range of 5 to 12 THz to be realized. With a GaN-based material, which is a typical one of the nitride semiconductors, the LO-phonon energy ELO is 90 meV, namely, about three times higher than that of GaAs. Because of the high LO-phonon energy, the phonon domain shifts to near 22 THz, which is equivalent to the energy of the LO-phonon, enabling the prevention of absorption due to electron-LO-phonon scattering in a frequency range of 5 to 12 THz. Furthermore, a higher energy of electron-LO-phonon scattering is advantageous also in that operation at high temperature can be expected.
It is noted that a theoretical calculation result is disclosed in the case that a super lattice having two well layers of GaN and two barrier layers of AlGaN in each unit corresponding to one period is adopted (see for example, Patent Literature 1, claim 2 therein). However, when a crystal lattice is grown on a polarized surface, which is important in terms of crystal growth, it is shown that a gain takes on negative values at energies corresponding to frequencies of 5 THz or above, or about 20 meV or more. Therefore, lasing operation at frequencies over 5 THz cannot be expected (see for example, Patent Literature 1, FIG. 17). In addition, in this disclosure, actual operation is not predicted specifically. For example, lasing frequencies to be operated are not identified.